An example of a device including an optical bistable switch which is turned on and off with optical signals is the optical memory device described in the Japanese Patent Application #60-15178 (Disclosure #62-13086). The circuit of the optical memory device is shown in FIG. 6. In the circuit, the series connection of a light-emitting device 1 and a first phototransistor 2 constitutes an optical bistable switch 3. The optical bistable switch 3 is operated as follows: (1) While the bias voltage is higher than the holding voltage, which is the minimum voltage for maintaining the on-state is applied to the switch, current does not flow through the first phototransistor 2 and the light-emitting device 1 does not emit output light (off-state). (2) When input light is incident upon the first phototransistor 2 and the bias voltage is maintained above the holding voltage, the current starts to flow and the light-emitting device 1 emits output light (on-state). (3) When the illumination of input light is stopped and the bias voltage is maintained above the holding voltage, the phototransistor 2 detects the light emitted from the light-emitting device 1, and the on-state is maintained. (4) When the bias voltage is decreased beneath the holding voltage, the switch is turned off.
In the optical memory device, a second phototransistor 4 is connected in parallel to the optical bistable switch 3, and the optical bistable switch 3 can be turned off by illuminating the second phototransistor 4. This is because the current flowing through the second phototransistor 4 increases the voltage drop in the load resistor 5 and decreases the voltage applied to the optical bistable switch 3. Accordingly, the optical memory device can be turned on with set light incident upon the first phototransistor 2, and turned off with reset light incident upon the second phototransistor 4 while the bias voltage higher than the holding voltage is continuously applied.
The concrete structure of this optical memory device is shown in FIG. 7. The second phototransistor 4 is constructed with a second collector layer 7, a second base layer 8, and an emitter layer 9 on a semiconductor substrate 6; the first phototransistor 2 is with the emitter layer 9, a first base layer 10, and a first collector layer 11; and the light-emitting device 1 is with the first collector layer 11, an active layer 12, and a cladding layer 13. The cathode of the light-emitting device 1 and the collector of the first phototransistor 2 are connected electrically inside the device since they are made of the identical semiconductor layer (the first collector layer 11). In the same manner, the emitter of the first phototransistor 2 and the emitter of the second phototransistor 4 are connected. The anode of the light-emitting device 1 (the cladding layer 13) and the collector of the second phototransistor 4 (the second collector layer 7) are coupled with an external wiring 14 and connected to the anode of a power supply 15 through a load resistor 5. The internally connected emitters of the first phototransistor 2 and the second phototransistor 4 (the emitter layer 9) are connected to the cathode of the power supply 15.
In this structure, the optical signals for turning on and off the optical bistable switch 3 are introduced through the semiconductor substrate 6. The set light and the reset light are distinguished by their wavelengths. The energy bandgap of the second base layer is wider than that of the first base layer. Setting .lambda..sub.1 and .lambda..sub.2 as the wavelengths corresponding to the bandgaps of the first and the second base layers, the input light with a wavelength .lambda..sub.ON satisfying .lambda..sub.2 &lt;.lambda..sub.ON .ltoreq..lambda..sub.1 is transmitted through the second base layer and absorbed in the first base layer. Therefore, the optical bistable switch is turned on with the input light having a wavelength .lambda..sub.ON. On the other hand, the input light with a wavelength .lambda..sub.OFF satisfying .lambda..sub.OFF .ltoreq..lambda..sub.2 is absorbed in the first base layer and turns off the optical bistable switch.
In the optical memory device mentioned above, two light sources are required to turn on and off the optical bistable switch since the wavelengths of the set light and the reset light are different. Two light sources are still necessary for the optoelectronic integrated circuit described in the Japanese Patent Application #1-322147 (Disclosure #3-181920) where the first and the second phototransistors are formed on separate areas of a semiconductor substrate to distinguish the set light and the reset light by spatial location and not by wavelength. On the other hand, the U.S. Pat. No. 5,095,200 (Mar. 10, 1992), incorporated herein by reference, describes an optoelectronic memory device in which the optical bistable switch can be turned on and off with a single optical beam.
In this optoelectronic memory device, the first and the second phototransistors are formed on separate areas of a semiconductor substrate, which is similar to the optoelectronic integrated circuit described in the Japanese Patent Application #1-322147. However, the input light is incident upon both the first and the second phototransistors simultaneously by using a large-diameter beam as the input light. There are two cases in which the optical bistable switch is turned on and off when the input light is incident upon the first and the second phototransistors simultaneously. Whether the optical bistable switch is turned on or off depends on the equivalent gains of the first and the second phototransistors, the value of the load resistance, and the bias voltage applied to the whole circuit. If the equivalent gains of the phototransistors and the resistance of the load resistor are set properly, the optical bistable switch can be turned on and off with a single optical beam by varying the bias voltage.
The optoelectronic memory device described above has the advantage that the optical bistable switch can be turned on and off with the identical input light. However, the optical bistable switch is not fully controlled with the optical signal since the bias voltage should be changed to distinguish the set and the reset operations. Furthermore, the equivalent gain of the first phototransistor affected by the optical positive feedback is much larger than the gain of the second phototransistor when the same structure for the first and the second phototransistors is used. To compensate for this imbalance, the portion of the input optical power incident upon the second phototransistor should be made larger than that upon the first phototransistor, which requires precise optical alignment for the input optical beam.